FIELD OF THE INVENTION
The invention relates to a storage cage for the storage and transport of a plurality of fuel assemblies, in particular fuel assemblies which have been used in a nuclear reactor for the release of energy by nuclear fission in fissionable material that is contained in the fuel assemblies.
Both the storage and the possibly necessary transport of fuel assemblies, in particular after the fuel assemblies have been used in a nuclear reactor, necessitate an abundance of measures arising from reasons of radiation protection, a need to dissipate heat originating from radioactive decays in the fuel assemblies and a requirement of preventing the formation of an accumulation of fissionable material in which an automatic chain reaction of nuclear fissions could occur.
The first-mentioned reason necessitates a careful shielding of an irradiated fuel assembly, the second-mentioned reason demands special measures for heat dissipation and the third-mentioned reason necessitates that a multiplicity of fuel assemblies always be gathered together in a narrow space only with an appropriate quantity of neutron-absorbing material.
German Patent DE 32 21 810 C2 discloses a device for the storage of irradiated fuel assemblies. The device is to be disposed in a nuclear power station in the vicinity of the nuclear reactor and inside a guard shield surrounding the nuclear reactor. The device includes a storage pond filled with light water and a rack which is disposed in the storage pond and in which the irradiated fuel assemblies can be stored. The rack has a baseplate, on which casings made of boron-containing steel are mounted in a configuration in the manner of a honeycomb. Each casing serves for receiving a fuel assembly. The shielding of the fuel assemblies is guaranteed, in particular, through the use of water. The water also serves, together with the material of the casings, for the absorption of neutrons, in order to reliably rule out the buildup of a chain reaction.
A method and a storage device for the treatment of spent fuel assemblies from nuclear power stations are disclosed in German Published, Non-Prosecuted Patent Application DE 34 00 929 A1. According to that publication, fuel assemblies which are to be removed from a nuclear power station are sealingly enclosed in cans inside the guard shield of the nuclear reactor. Only sealingly closed and, where appropriate, externally decontaminated cans are brought out of the guard shield and stored in an external storage device which is equipped with air cooling for the dissipation of heat energy from the fuel assemblies.
German Published, Non-Prosecuted Patent Application DE 28 40 594 A, like German Patent DE 32 21 810 C, relates to a storage rack for fuel assemblies to be placed in a water pond. The storage rack is composed of rack parts which are disposed next to one another and, where appropriate, also one above the other. A rack part has projections and recesses at the edge of its cross section, which can be fitted together with corresponding recesses or projections in adjacent rack parts and which, in particular, form supporting surfaces between the rack parts. The rack formed from the rack parts, which can be stacked, in particular, in the manner of beer crates, can absorb not only vertically acting, but also horizontally acting forces.
Published European Patent Application 0 385 186 A1 describes a fuel assembly storage rack with an outer frame which has transport lugs at its upper end and which includes a horizontal support plate at its lower end. Vertical tubes having a baseplate are disposed in the outer frame and through the use of which they are fastened to the support plate. Vertical wall surfaces of the tubes run parallel to one another and to the outer frame. In order to ensure the passage of cooling water, the baseplates or the vertical wall surfaces have perforations which are matched with corresponding perforations in the outer frame. The transport lugs serve for lifting the storage rack through the use of a lifting appliance into a fuel assembly storage pond which is provided and for positioning it there.
U.S. Pat. No. 4,960,560 specifies a fuel assembly storage rack with a baseplate and cells being fastened to the base-plate, being extended perpendicularly to the latter and being intended for receiving fuel assemblies. The baseplate contains an aperture for each cell, for the entry of cooling liquid. In order to lift the storage rack, after its assembly, into the intended location in a fuel assembly storage pond, some of the apertures are formed in such a way that an appliance for lifting the storage rack can engage into them. Through the use of the appliance, the storage rack and the fuel assemblies which are disposed therein solely vertically relative to the baseplate, can be lifted into different positions within the fuel assembly storage pond.
The known possibilities for the storage and transport of fuel assemblies always involve the need, where appropriate, to reload fuel assemblies individually from a first storage device into a second storage device, for example from a rack into a transport container. In view of the special requirements to be placed on the handling of such fuel assemblies, that means that handling should always take place only in a specially shielded environment. That also results in a very high outlay which is extremely undesirable, not the least for reasons of radiation protection, since individual fuel assemblies have to be moved for each reloading operation.